The present invention relates to an image-reading lens for image reading which is suitable for capturing a manuscript image in image-reading devices such as an image scanner, and the like, and image-forming apparatuses such as a fax machine, a digital copier, and the like. In particular, the present invention relates to a small and high-performance image-reading lens having a small number of lenses, relates to an image-reading device using such an image-reading lens, and relates to an image-forming apparatus using such an image-reading device.
In an image-reading device in an image scanner, a fax machine, a digital copier, and the like used for manuscript image reading, an optical image of a manuscript to be read is reduced and formed by an image-reading lens for reading, this reduced optical image is imaged by a solid-state image sensor such as a CCD (Charge-Coupled Device) image sensor, and image information is converted to electronic image data. Additionally, in order to read manuscript information in color, a solid-state image sensor using a so-called 3-line CCD image sensor in which light-receiving elements each having, for example, red, green, and blue filters are arranged in three lines is structured, on its light-receiving surface, a reduced optical image of a manuscript is formed, three-primary-color separation is performed, and color image information is converted to signals.
In the above image-reading lens, generally, high contrast in a high spatial frequency region on an image plane, and an almost 100% aperture efficiency to a peripheral portion of an angle of view are demanded. Additionally, in order to favorably read a color manuscript, on a light-receiving surface, image-forming positions of red, green, and blue colors are needed to coincide in an optical axis direction, and chromatic aberration of each color has to be corrected excellently.
Traditionally, a so-called Gauss-type image-reading lens widely used as such an image-reading lens makes it possible to realize a relatively large diameter, and high resolution. However, in order to realize high performance which has been demanded recently with a Gauss-type lens structure, the number of lenses equal to or more than 6 lenses such as a lens structure having 6 lenses in 4 groups, a lens structure having 8 lenses in 5 groups, and the like is needed.
On the other hand, Japanese Patent Number 3939908 discloses an image-reading lens in which the number of lenses is smaller compared to a Gauss-type lens and which has performance equal to or higher than that of the Gauss-type lens, and has a lens structure having 3 to 5 lenses.
That is, the image-reading lens disclosed in Japanese Patent Number 3939908 realizes an optical system in which the number of lenses in the lens structure is 3 at minimum, a lens which is located on a most image side has negative power, and the lens is located adjacent to an image plane, which makes it easy to correct various aberrations and possible to realize high performance.